1. Field of the Invention
This invention relates to an aqueous anodizing solution for anodizing an article of magnesium or magnesium-base alloy to provide the surface of the article with corrosion resistance, abrasion resistance, and ornamentality. This invention further relates to a process for coloring an article of magnesium or magnesium-base alloy.
2. Description of Prior Arts
Magnesium and magnesium-base alloy are light in weight and favorable in the mechanical characteristics. However, magnesium and magnesium-base alloy are chemically active so that these are readily corroded. For this reason, an article of magnesium or magnesium alloy is generally employed after having been subjected to a certain surface treatment to make it anti-corrosive. Heretofore, various surface treatments have been proposed and some of them have been actually employed.
The known surface treatments for magnesium and magnesium-base alloy are broadly divided into two groups, namely, a chemical conversion and an anodic oxidation.
Representative examples of the chemical conversion include processes described in JIS (Japanese Industrial Standard) H 8651(1978) at Groups 1 to 4, and Group 7. Representative examples of the anodic oxidation include processes described in JIS H 8651(1978) at Groups 5 and 6 (formulation of the anodizing solution of Group 5: ammonium nitrate, sodium bichromate and aqueous ammonia, and formulation of the anodizing solution of Group 6: sodium hydroxide, ethylene glycol, sodium oxalate, sodium bichromate and acidic sodium fluoride); HAE process described in MIL Standard (MIL-M-45202B, formulation of the anodizing solution: potassium hydroxide, potassium fluoride, sodium phosphate, aluminum hydroxide, and potassium permanganate); and Dow-17 process (formulation of the anodizing solution: ammonium hydrogenfluoride, sodium bichromate, and phosphoric acid).
The above-described conventional processes, however, all have disadvantageous features. For instance, although the chemical conversion process is simple in its procedure, the converted surface of an article of magnesium or magnesium-base alloy does not have sufficient corrosion resistance. Thus, the chemical conversion is employed simply for tentative anti-corrosive treatment.
In contrast, the anodic oxidation is generally employed for a long-term anti-corrosive treatement. Among the known anodic oxidation processes, the HAE process and Dow-17 process are effective to impart to the metal surface high corrosion resistance. Nevertheless, improvements are still desired in the processes particularly in the aspects of level of corrosion resistance and ornamentality of the article provided thereby. Moreover, these processes have disadvantageous features in that the surface of an article of magnesium or magnesium-base alloy is automatically colored to show dark brown or dark green face. Although the magnesium or magnesium-base alloy article thus colored in the corrosion inhibiting treatment shows no substantial problem as far as chemical and physical properties are concerned, said article is not willingly accepted as material of commercial goods. More in detail, the article of magnesium or magnesium-base alloy is incorporated into commercial goods generally upon having been colored on the surface to match with the object of the goods to enhance the ornamental or decorative image thereof. The originally dark-colored surface is difficultly colored to show an optional color or an optional hue even though a chemical or electrochemical coloring method is or a painting method applied to the surface.
For obviating the above-described problem, Japanese Patent Provisional Publication 55(1980)-76094 proposes a process for coloring a surface of an article of magnesium or magnesium-base alloy, which comprises a step of treating said surface through an electrolytic treatment (anodic oxidation) in an alkaline electrolytic bath containing trisodium phosphate and an organic acid (or an inorganic acid) or a salt thereof and a subsequent step of coloring the so treated surface of the article with a dye for coloring aluminum (i.e., anodized aluminum-coloring dye). This process, however, requires an additional decolorization-preventing process for keeping the dye on the surface. Therefore, this process still has a disadvantageous feature that the procedures are complicated.